Julián Castro, the former housing secretary and former mayor of San Antonio, announced on Saturday that he would run for president, one of the most high-profile Latino Democrats ever to seek the party’s nomination.

His first campaign stop will be in Puerto Rico, where he will speak on Monday at the Latino Victory Fund’s annual summit and meet with residents still struggling to recover from Hurricane Maria. Later in the week, his campaign said, he will go to New Hampshire.

“When my grandmother got here almost a hundred years ago, I’m sure that she never could have imagined that just two generations later, one of her grandsons would be serving as a member of the United States Congress and the other would be standing with you here today to say these words: I am a candidate for president of the United States of America,” Mr. Castro said Saturday at the Plaza Guadalupe amphitheater in San Antonio.

Mr. Castro’s announcement had been expected for several weeks. He established an exploratory committee in December, two months after publishing a memoir, “An Unlikely Journey” — a familiar path for presidential candidates who want to play up their life stories and qualifications and, perhaps, get ahead of their biggest vulnerabilities. This month he also visited two of the early caucus and primary states, Iowa and Nevada, where he emphasized health care (he supports Medicare for All) and affordable housing.

He now joins Senator Elizabeth Warren of Massachusetts, Representative Tulsi Gabbard of Hawaii and former Representative John Delaney of Maryland on the still-short list of Democrats who have said definitively that they will seek for the party’s 2020 nomination. That list is expected to grow considerably in the coming weeks and months.

[Read about what Mr. Castro and the other early entrants will be doing in the early days of the 2020 campaign.]

Mr. Castro, 44, was raised in San Antonio in a politically active family. His mother, Rosie Castro, was an activist with the Mexican-American political party La Raza Unida and frequently took Julián and his twin brother, Joaquin — now a congressman — to rallies and meetings. Joaquin Castro will be the chairman of Julián’s campaign.

At the age of 26, Mr. Castro became San Antonio’s youngest City Council member, and after one unsuccessful campaign for mayor in 2005, he was elected to the city’s top job in 2009. In 2012, he delivered the keynote address at the Democratic National Convention — the same platform that catapulted Barack Obama, then a little-known state senator, to national prominence in 2004. Mr. Obama, as president, later chose him to lead the Department of Housing and Urban Development.

Many of the likely 2020 candidates are better known, but Mr. Castro has brushed this off.

“In my whole life, I don’t think I’ve ever started out as the front-runner,” he told The New York Times last month. “I grew up in a neighborhood where nobody growing up there was the front-runner at anything. So I’m not going to concern myself with who people think of as the front-runner and who they don’t.”

Among the Democrats frequently mentioned as 2020 contenders, he is the only Latino, and also one of the youngest. If elected, he would be the third-youngest person ever to become president, after Theodore Roosevelt (42) and John F. Kennedy (43).

The full field of candidates could include former Vice President Joseph R. Biden Jr.; Senators Cory Booker of New Jersey, Kirsten Gillibrand of New York, Kamala Harris of California, Amy Klobuchar of Minnesota and Bernie Sanders of Vermont; and former Representative Beto O’Rourke of Texas, who ran a high-profile but unsuccessful campaign against Senator Ted Cruz last year.